The present invention relates generally to the field of avionics for airborne collision avoidance systems (CAS).
Spurred by the collision of two airliners over the Grand Canyon in 1956, the airlines initiated a study of collision avoidance concepts. By the late 1980's, a system for airborne collision avoidance was developed with the cooperation of the airlines, the aviation industry, and the Federal Aviation Administration (FAA). The system, referred to as Traffic Alert and Collision Avoidance System II (TCAS II) was mandated by Congress to be installed on most commercial aircraft by the early 1990's. A chronology of the development of airborne collision avoidance systems can be found in "Introduction to TCAS II," printed by the Federal Aviation Administration of the U.S. Department of Transportation, March 1990.
The development of an effective airborne CAS has been the goal of the aviation community for many years. Airborne collision avoidance systems provide protection from collisions with other aircraft and are independent of ground based air traffic control. As is well appreciated in the aviation industry, avoiding such collisions with other aircraft is a very important endeavor. Furthermore, collision avoidance is a problem for both military and commercial aircraft alike. In addition, a large, simultaneous number of TCAS interrogations from close-in formation aircraft members generate significant radio frequency (RF) interference and could potentially degrade the effectiveness of maintaining precise position/separation criteria with respect to other aircraft and obstacles. Therefore, to promote the safety of air travel, systems that avoid collision with other aircraft are highly desirable.
Referring to FIG. 1, there is shown a block diagram of a conventional TCAS system. Shown in FIG. 1 are TCAS directional antenna 10, TCAS omni-directional antenna 11, and TCAS computer unit 12, which includes receiver 12A, transmitter 12B, and processor 12C. Also shown are aural annunciator 13, traffic advisory (TA) display 14, and resolution advisory (RA) displays 15. Alternatively, the TA and RA displays are combined into one display (not shown). The transponder is comprised of transponder unit 16A, control panel 16B, and transponder antennas 16C and 16D. The TCAS and transponder operate together to function as a collision avoidance system. Those skilled in the art understand that this is merely illustrative of a conventional TCAS. For example, many other configurations are possible such as replacing omni-directional antenna 11 with a directional antenna as is known to those skilled in the art. The operation of TCAS and its various components are well known to those skilled in the art and are not necessary for understanding the present invention.
In a TCAS system, both the interrogator and transponder are airborne and provide a means for communication between aircraft. The transponder responds to the query by transmitting a reply that is received and processed by the interrogator. Generally, the interrogator includes a receiver, an analog to digital converter (A/D), a video quantizer, a leading edge detector, and a decoder. The reply received by the interrogator includes a series of information pulses that may identify the aircraft, or contain altitude or other information. The reply is a pulse position modulated (PPM) signal that is transmitted in either an Air Traffic Control Radar Beacon System (ATCRBS) format or in a Mode-Select (Mode-S) format. Other replies are possible as is known to those skilled in the art.
A TCAS II equipped aircraft can monitor other aircraft within approximately a 20-mile radius of the TCAS II equipped aircraft. (see, e.g., Brettner et al., U.S. Pat. No. 5,805,111, Method and Apparatus for Accomplishing Extended Range TCAS, which describes an extended range TCAS.) When an intruding aircraft is determined to be a threat, the TCAS II system alerts the pilot to the danger and gives the pilot bearing and distance to the intruding aircraft. If the threat is not resolved and a collision or near miss is probable, then the TCAS II system advises the pilot to take evasive action by, for example, climbing or descending to avoid a collision.
The TCAS II system, which is currently in operation on many commercial and military aircraft, is very effective in providing midair collision avoidance in civil Air Traffic Control (ATC) airspace in determining the range, altitude, and bearing with other aircraft equipped with Mode S/ATCRBS transponders. It monitors the trajectory of these aircraft for the purpose of determining if any of them constitute a potential collision hazard. The system is responsible for estimating the projected intruder track and determining if a potential conflict exists. If a conflict is detected, the system displays an advisory to the pilot. The system also provides guidance for vertical avoidance maneuver, known as Resolution Advisories (RAs). Complementary avoidance maneuvers between two TCAS equipped aircraft are ensured by automatic coordination of mutual intentions with the other aircraft through the Mode S transponders and associated TCAS.
However, the TCAS II (or other TCAS units) originally was not designed to handle unique mission capabilities, which would be required, for example, by military aircraft. Examples of such unique mission capabilities are: operate in a tactical environment (tactical speed and dynamic maneuvers), perform highly dynamic, close-in formation flight, rendezvous, and air refueling. At the same time, the system must effectively detect and avoid midair collision situation.